That's the finding of research released yesterday in the journal Proceedings of the National Academy of Sciences.

The study analyzed charcoal found in sediment cores from 14 lakes in the Yukon Flats region of the state to determine the frequency of past fires.

Over the last 3,000 years, the average fire frequency in the area was about 10 fires per thousand years.

In the last 50 years, the fire frequency has nearly doubled, up to 20 every thousand years, or one fire every 50 years.

Climate scientists care about what might happen in boreal forests because they cover 10 percent of land surface and store a lot of carbon in their soil, said Ryan Kelly, an ecologist and doctoral candidate at the University of Illinois who was lead author of the study.

"It is a significant player in the global carbon cycle," Kelly said. "When the forests burn, the carbon goes into the atmosphere. If they burn more frequently, they are releasing more carbon, and storing less."

The researchers were particularly interested in comparing fire frequency from the recent past with that of a period called the Medieval Climate Anomaly, when conditions were warm and dry, similar to recent decades.

Forest may self-regulate
When they did this, they found a surprising—and possibly hopeful—result. They noted that fire frequency was also high about 1,000 years ago, during the anomaly.

But pollen records the researchers analyzed show that during that time of increased fire, the forest responded by changing the vegetation that regrew after the fires. The trees shifted from evergreen to deciduous. The deciduous trees, like aspen and birch, did not burn as easily, and this slowed down the fire frequency.

"So that's really interesting to us as ecologists, because it is a mechanism by which ecosystems are kind of regulating themselves," he said.

Jennifer Marlon, a scientist at the Yale School of Forestry and Environmental Studies who studies wildfires and climate change, said the study was unique because it used a lot of records from one location. "Their conclusions were very robust because of the scope and scale of the study," Marlon said.

The findings were also unique because the researchers were able to so closely link historical records with what is happening in the present and what might happen in the future, she said. "Using the paleo records, long historical records like this, it is fairly rare to be able to connect it so closely to what is happening today."

The fact that the number of forest fires in a past warm period was reduced by the regrowth of different trees might offer some hope for reduced fire frequency in the future, even though the frequency in recent decades has been high, Kelly said.

This idea could be validated by modeling studies. It also might play out in the next few decades, as scientists watch, Kelly said.

Whether the regulating mechanism of deciduous trees kicks in could depend on how much that region warms, said Philip Higuera, an ecologist from the University of Idaho and a co-author on the study.

"To me, the key thing that that hangs on is how much we turn that temperature knob up," said Higuera.